1. Field of the Invention
The present invention relates to the recovery of precious metals from carbonaceous ores. More particularly, the invention concerns improved leaching techniques for these ores.
2. Description of the Prior Art
Gold is one of the rarest metals on earth. Gold ores can be categorized into two types: free milling and refractory. Free milling ores are those that can be processed by simple gravity techniques or direct cyanidation. Refractory ores, on the other hand, are difficult to process. Refractory ore resources can consist of ores, flotation concentrates, mill tailings, and other reserves. In the past, refractory ores have required pre-cyanidation treatments to liberate the gold. The difficulty of processing refractory gold ores is attributable to their mineralogy.
A large number of refractory ores consist of ores with a precious metal such as gold occluded in iron sulfide particles. The iron sulfide particles consist principally of pyrite and arsenopyrite. If the gold remains occluded, even after fine milling of these ores, then the sulfides must be oxidized to liberate the encapsulated precious metal and make it amenable to a leaching agent (or lixiviant).
Carbonaceous gold ores represent a unique class of refractory ores. Not only is gold sometimes found encapsulated in sulfide minerals in these ores, but these ores also contain carbonaceous matter that interferes with recovery by cyanidation. Gold in carbonaceous ores, therefore, can be associated with sulfide minerals, carbonaceous matter, and/or siliceous minerals. P. Afenya, Treatment of Carbonaceous Refractory Gold Ores, Minerals Engineering, Vol. 4, Nos 7-11, pp 1043-55, 1991, hereby incorporated by reference. The distribution of gold in these mineral groups can vary considerably from ore to ore.
Researchers have identified the carbonaceous matter in these ores as containing (1) an activated carbon component capable of adsorbing gold-chloride complexes and gold-cyanide complexes from solution, (2) a mixture of high molecular weight hydrocarbons usually associated with the activated carbon components; and (3) an organic acid, similar to humic acid containing functional groups capable of interacting with gold complexes to form organic gold compounds. P. Afenya, Treatment of Carbonaceous Refractory Gold Ores, Minerals Engineering, Vol. 4, pp. 1043-1055, 1991. hereby incorporated by reference; W. Guay, The Treatment of Refractory Gold Ores Containing Carbonaceous Material and Sulfides, Society of Mining Engineers of AIME, 81-34, pp. 1-4, 1981, hereby incorporated by reference.
Carbonaceous matter, can therefore directly or indirectly interfere with lixiviation. Direct interference with lixiviation is ascribed to either occlusion of the gold within the carbonaceous material or formation of a stable gold-carbon complex similar to a chelate. The more common problem with these ores, however, is indirect interference. This occurs when the gold-lixiviant complex formed during lixiviation is sorbed by the native carbonaceous material and, therefore, is no longer available for recovery from solution. This phenomenon is called preg-robbing.
Preg-robbing is frequently associated with the use of cyanide as the lixiviant. However, it also occurs with gold-lixiviant complexes other than autocyanide.
Certain clay materials such as illite, kaolin, and montmorillonite are also known to preg-robbingly adsorb the gold-cyanide complex. Thus, the degree of preg-robbing exhibited by an ore depends on the amount of carbonaceous matter and preg-robbing clay materials in the ore. As used herein, it should be understood that carbonaceous component and carbonaceous matter also refer to preg-robbing clays, because the preg-robbing properties of these materials are functionally similar to that of the actual carbonaceous matter in the ore.
While preg-robbing is most frequently associated with cyanidation processes, the preg-robbing phenomenon is also known to occur with other gold-lixiviant complexes such as gold-chloride. The inventor has even experienced preg-robbing of gold-thiourea complexes while using a thiourea lixiviant.
Carbonaceous ores vary significantly from deposit to deposit, and even within deposits, in the amount of carbonaceous matter they contain. These ores have been reported to contain from approximately 0.2% carbon to as much as 5% carbon. P. Afenya, Treatment of Carbonaceous Refractory Gold Ores, Minerals Engineering, Vol. 4, pp. 1043-1055, 1991.
If P represents the preg-robbing component of the ore, V represents a valuable mineral component (i.e., gold, silver, or platinum), and G represents the gangue materials in the ore, then preg-robbing may be illustrated by the following general formula: ##STR1## Wherein V.sub.1 represents the precious metal closely associated with the preg-robbing material in the ores, V.sub.2 represents the precious metal associated with gangue material, V.sub.x represents the precious metal preg-robbingly removed from the lixiviant solution, V.sub.y represents the precious metal-lixiviant complexes remaining in solution, and V.sub.2-(x+y) represents the amount of precious metal remaining associated with the gangue material after lixiviation.
Thus, the amount of precious metal that is associated with the preg-robbing component of the ore after lixiviation is equal to the amount of precious metal originally associated with the preg-robbing component of the ore plus the amount that is preg-robbingly removed from the lixiviant solution (V.sub.x). The amount of precious metal remaining associated with the gangue material (V.sub.2-(x+y)) is equal to the original amount of precious metal (V.sub.2) minus the amount of precious metal dissolved by the lixiviant (V.sub.x +V.sub.y).
A number of techniques have been developed for processing refractory carbonaceous gold ores. These techniques include flotation, blanking, carbon in leach, roasting, chemical oxidation, and bacterial leaching. Roasting and oxidation by chlorination are the two methods that are most developed and applicable for treating carbon-bearing ores. The others may play some role in the future or are often confused with methods for processing carbonaceous ores, even within the mining industry, when they are really more suited to treating refractory sulfidic ores. The various techniques are described below: